1. Field of the Invention
The present invention relates to photoconductive organic pigment, a photographic organic pigment dispersion liquid, and an electrophotographic photoreceptor and an electrophotographic device using the same.
2. Description of the Related Art
As electrophotographic photoreceptors used in monochrome and full-color copiers, printers, facsimiles, digital copiers and the like, inorganic and organic photoreceptors are known. Among them, organic photoreceptors are principally used because they are not only environmentally friendly, but also possess advantages such as high productivity and low cost. Further, in the organic photoreceptors, photosensitivity can be controlled by selecting materials such as a charge generating material, a binder resin, a solvent and a sensitizer, and thus these materials are the subjects of extensive study.
Meanwhile, a laminated electrophotographic photoreceptor having a charge generating layer and a charge transporting layer as photosensitive layers has been proposed.
It is essential that this laminated electrophotographic photoreceptor possesses the required light sensitivity, image characteristics, shelf stability and the like to meet the needs of the electrophotographic process to which it is applied, and these characteristics are affected by the light sensitivity, chemical and physical stability and dispersibility of the charge generating material.
The charge generating material used in electrophotographic photoreceptors includes known photoconductive organic pigments such as polycyclic quinone pigments, perylene pigments, azo pigments, indigo pigments, quinacridone pigments and phthalocyanine pigments. Organic pigments can be synthesized more easily than inorganic materials, and can also be selected from a broader range of compounds exhibiting photoconductivity in a suitable wavelength range, and thus, a large number of photoconductive organic pigments have been proposed.
When the above-mentioned photoconductive organic pigments are used as the charge generating material, crude pigment crystals obtained by various synthesis methods are subjected to milling treatment, acid pasting treatment, solvent treatment and/or heat treatment thus changing their crystal form and regulating their particle diameter. By controlling the particle diameter, the sensitivity required for the charge generating material, and the photoreceptor characteristics such as light sensitivity, charging property, dark decay, environmental characteristics and cycle characteristics and the dispersibility, suitable coating and the like can be obtained in the production process.
Generally, the sensitivity of an electrophotographic photoreceptor using the photoconductive organic pigment as the charge generating material is almost always determined by the pigment used, and thus, selecting a pigment possessing the sensitivity required for the electrophotographic process is necessary in designing the electrophotographic photoreceptor. However, the required light sensitivity of the electrophotographic process does not necessarily conform with the light sensitivity of the electrophotographic photoreceptor, and problems may arise such as the thickening and thinning of thin lines, blurring and insufficient density. Hence, in order to achieve the formation of high-quality images, there is a limit to the selection of charge generating materials. Further, when highly light sensitive electrophotographic photoreceptors are used for small laser printers such as those widely used in homes or offices or for full-color printers/copiers of which high resolution is required, problems arise such as deterioration in resolution and in halftone reproduction, so there is a limit to the direct use of highly light sensitive pigments as the charge generating material.
When a charge generating material is dispersed in a resin, there are known methods wherein a binder resin or solvent used is changed or a mixing ratio of a pigment to resin is changed in order to regulate the sensitivity of the electrophotographic photoreceptor within a desired range, but these methods are subject to restriction on the structure or production of the photoreceptor, thus limiting the usable materials, so the actually required sensitivity is difficult to attain.
The regulation of sensitivity by using a mixture of a plurality of pigments has also been reported. For example, Japanese Patent Application Laid-Open (JP-A) No. 62-272272 describes use of α-type and β-type titanyl phthalocyanine pigments, and JP-A No. 2-183261 describes that a titanyl phthalocyanine pigment having a crystal form giving diffraction peaks at 9.6°, 11.7°, 24.1° and 27.2° in the Bragg angle (2θ±0.2°) is mixed with a titanyl phthalocyanine pigment having a crystal form giving a peak at 6.9°, 15.5° and 23.4° and it is known that titanyl phthalocyanine pigments different in crystal form are mixed in a different ratio to regulate the light sensitivity. Further, JP-A No. 2-280169 describes that different kinds of phthalocyanine pigments are mixed with the titanyl phthalocyanine pigment to regulate the sensitivity.
However, the range of the regulated sensitivity of the electrophotographic photoreceptor indicated in the above-listed publications is not always satisfactory, and the sensitivity varies depending on the pigment lot, making sensitivity regulation difficult. When the electrophotographic photoreceptor is used in which a charge generating material is dispersed in a resin, there are problems such as unsatisfactory dispersibility and shelf stability of the dispersion in practical use, a significant change in electric potential upon repeated use, and a significant change in characteristics in high- or low-humidity environments. Further, there are problems such as complicated production processes and higher cost. Accordingly, there is a need to truly understand sensitivity-regulating factors of photoconductive organic pigments used in the photoreceptor and to obtain photoconductive organic pigments capable of coping with demand for various light sensitivities.